1. Field of the Invention
The present invention relates to a color filter used for color sensors and for various display elements such as a CCD camera and liquid crystal display elements and to a formation technology of the color filter, as well as to a method for manufacturing of a color layer and a black matrix. In particular, the present invention relates to a novel method for manufacturing a color filter in which a color layer and a black matrix with high resolution can be simply formed without using a photolithographic process, to an apparatus used in this method and to a highly smoothed color filter having a transparent semiconductor film therein.
2. Description of the Related Art
Presently known methods for the manufacturing of a color filter include (1) a dyeing method, (2) a pigment dispersion method, (3) a printing method, (4) an ink jet method and (5) an electrodeposition method.
(1) The dyeing method involves forming an aqueous polymer to be dyed on a glass substrate, forming a desired pattern on the substrate through photolithographic steps and in succession dipping the substrate in a dye solution to obtain a colored pattern. These processes are repeated three times to obtain R. G. and B. (Red, Green and Blue) color filter layers. The resulting color filter has high transmittance and a great variety of hues and reaches a high technological level of perfection. Therefore, recently, this method has been used very often in making color solid charge coupled devices (CCD). However, the resulting color filter has deteriorated light resistance because a dye is used and this method involves many manufacturing steps. Hence, recently, the pigment dispersion method has been used instead of the dyeing method especially for making liquid crystal display elements (LCD).
(2) The pigment dispersion method has recently been the dominant method for manufacturing color filters. First, a resin layer is formed in which a pigment is dispersed on a glass substrate. A pattern is formed on the substrate thorough photolithographic steps. This procedure is repeated three times to obtain R. G. and B. color filter layers. This manufacturing method, while attaining a high level of technological perfection, requires many manufacturing steps and hence entails high cost.
(3) The printing method involves dispersing a pigment in a heat-curable resin and repeating printing three times to paint separately with R. G. and B., followed by heating to cure the resin, thereby preparing color filter layers. This method, while requiring no photolithographic steps, gives rise to a problem in terms of resolution and uniform film thickness.
(4) The ink jet method involves forming an ink acceptable layer of an aqueous polymer on a substrate and performing hydrophilic and hydrophobic treatments of the ink acceptable layer, followed by spraying ink using an ink jet method on a portion which has been made hydrophilic, painting separately with R. G. and B., thereby preparing color filter layers. This method also requires no photolithographic steps in the formation of R. G. and B. layers. However, it imparts inferior resolution. It is also inferior in positional accuracy because small droplets of ink scatter when spraying on an adjacent filter layer, with a high probability of color mixture.
(5) The electrodeposition method involves applying a voltage about as high as 70 V onto. a transparent electrode that has been patterned in advance, in an electrolyte wherein a pigment is dispersed in an aqueous polymer, to form an electrodeposition film and thereby perform electrodeposition coating. These processes are repeated three times to obtain R. G. and B. color filter layers. This method, however, requires forming a pattern in advance on a transparent electrode by means of photolithography, and using this transparent electrode as the electrode for electrodeposition. The shape of the pattern on the electrode is limited. Therefore, this method cannot be used for a TFT liquid crystal.
The present inventors have studied such an electrodeposition technology itself based on its principle and as a result, have perceived that there are, among aqueous color molecules, those which exhibit greatly different solubilities in water under different conditions, namely, oxidation conditions, neutral conditions and reduction conditions.
Examples of compounds having such properties are as follows. At a pH of 4 or more, Rose Bengale and eosin, which are fluorescein type dyes, are in a reduction condition so are soluble in water, but, below pH 4, these compounds are in a neutral condition so these compounds precipitate/sediment. It is also known that dye materials having a carboxyl group greatly vary in solubility according to the hydrogen ion concentration (pH), even without any structural changes. Specifically, ink jet dyes that are improved in water resistance are soluble in water at a pH of 6 or more, but precipitate below this pH. When these dyes are dissolved in pure water and an electrode is dipped in the solution to apply voltage, electrodeposition films consisting of these dye molecules are formed on the anode. An aqueous acrylic resin, which has a carboxyl group and is a type of polymer, is also soluble in water at a pH 6 or more, but precipitates below that pH. When an electrode is dipped in a solution, in which a pigment is dispersedin this polymer, to apply voltage, the pigment and the polymer precipitate on the anode to form an electrodeposition film wherein the pigment is mixed with the polymer. These electrodeposition films can be resulted in aqueous solutions by applying reverse voltage or by dipping these films in aqueous solutions whose pH is between 10 and 12. An oxazine type basic dye Cathilon Pure Blue 5GH (C.I. Basic Blue 3), which is a quinoneimine dye, and a thiazine type basic dye Methylene Blue (C.I. Basic Blue 9) are oxidized to develop a color at a pH of 10 or less, but a above this pH, are reduced, becoming insoluble and thereby precipitatig. When these dyes are dissolved in pure water and electrodes are dipped in the solution to apply voltage, electrodeposition films consisting of these dye molecules are formed on the cathodes. These dye electrodeposition films are restored to their original states and resulted by applying reverse voltage or by dipping these films in aqueous solutions whose pH is 8 or less.
In conventional electrodeposition technologies, the voltage required for forming an electrodeposition film is about as high as 70 V. Applying such a high voltage causes a Schottky barrier between a semiconductor and an electrolyte to be broken, with the result that no image can be formed. In addition, there have not been any semiconductors that can be used for forming transparent and practical color filters. In view of this situation, the patterning of a transparent electrode is required in the aforementioned conventional method for the manufacturing of a color filter that makes use of electrodeposition coating. This causes the pattern of color filters to be limited in shape.
There are methods proposed in which a dye is used for doping or dedoping of electroconductive polymers to form an image using light. In this case, it is possible to form an electroconductive film using only a dye without using an electroconductive polymer. However, the voltage required for forming an electrodeposition film using only a dye is larger than that in the case of using an electroconductive polymer as well. Meanwhile, the photovoltaic force is in order of about 0.6 V even in common Si. The photovoltaic force alone is insufficient to form an image. Accordingly, it is possible to consider applying bias voltage to raise the power. However, the Schottky barrier between a semiconductor and a solution, which is necessary for the creation of the photovoltaic force, is broken above a fixed voltage (the voltage is dependent on the bandgap of the semiconductor used). This limits applicable bias voltage. Therefore, the formation of an image in an aqueous solution using photovoltaic force is limited to using a photopolymerization reaction of an electroconductive polymer such as polypyrrole which is oxidized and reduced at 1.0 V or below. Also, the electrodeposition voltage is as high as 20 to 80 V and the oxidizing and reducing reaction of a polymer is utilized for the formation of an electrodeposition material according to the disclosures of Japanese Patent Application Laid-Open (JP-A) No. 5-119209 (entitled "METHOD FOR PRODUCING COLOR FILTER AND ELECTRODEPOSITION SUBSTRATE FOR PRODUCING COLOR FILTER") and JP-A No. 5-157905 (entitled "METHOD FOR PRODUCING COLOR FILTER"), which are well-known in this field. As is clear from the above, the voltage required for the electrodeposition of polymers generally known as a material for electrodeposition coating is greater than or equal to 10 V. To form an image, the photoconductive characteristics of, for example, ZnO.sub.2, which is used for electrophotography, are utilized. A practical material that is usable in a water-type solution and that can be handled with ease has not been found as of yet.
A color filter of only a color filter layer is scarcely used. Those in which each of the gaps between pixels is covered with a black matrix are used in general. For the formation of the black matrix, a photolithographic method is usually used. This is one of the primary causes of increased costs. In consideration of the structure, which includes R. G. and B. layers and the black matrix, a method has not yet been found which can produce a color filter having high resolution and high controllability and which requires no photolithographic steps and is reduced in the number of manufacturing steps. For instance, it is well-known that a large part of the costs for a liquid crystal color display device and the like is occupied by the cost of the color filter. This is largely due to low yield in the manufacturing of color filters with the result of high cost.
Among the color filters produced by each of the above-mentioned methods (1) to (5), those produced by the methods (1), (2), and (5), which use a photolithographic method, have irregularities formed on the substrate and on the color filter layer, which is formed by photolithography. Those produced by the methods (3) and (4) in which printing technologies are applied have irregularities on the surface of the ink layer. All these methods pose the problem of impaired surface smoothness. Even if a simple protective layer is formed on the surface, such a protective layer has a thickness insufficient to counteract the influence of the irregularities. If a smoothing treatment is not carried out, ideal smoothness cannot be achieved.